Most metal parts operate in an environment which eventually leads to corrosion or the creation of stress induced cracks, thereby reducing the useful life of such parts. It is known that peening the surface of metal parts can induce compressive residual surface stresses, thereby increasing the resistance of the part to fatigue, cracking and corrosion. Numerous methods exist which relate to peening the exterior surface of metal parts. However, these methods are not applicable to peening the internal surface of hollow parts because such methods fail to take into account the peculiar difficulties associated with peening the internal surface.
U.S. Pat. No. 2,460,657 addressed some of the distinctive characteristics associated with peening the internal surface of a hollow part. Specifically, that patent taught that vibrating the part produced repeated impact between the peening elements and the internal surface of the hollow part. Vibrating the part, however, may be impractical in many instances due to the size and shape of the part. Vibrating the entire part also limits the ability to control or vary the peening intensity at different points along the internal surface. Furthermore, vibrating the part may itself increase the stress within the part, thereby initiating the formation of cracks and negating the benefit attained from peening.